1. Field of the Invention
The present invention generally relates to the field of improvement of combustion conditions for fuel combustors. More particularly, the present invention relates to the field of air-fuel injection systems for stable combustion.
2. Description of the Prior Art
The formation of NOx and SOx during combustion has been a long existing problem in commercial fuel combustors. Preventing the formation of NOx and SOx during combustion requires that initial combustion be very fuel-rich or very fuel-lean, sometimes beyond the rich and lean flammable limits. However, these approaches present difficult combustion conditions to maintain stable ignition and to avoid lift- or blow-off of the flame, and to prevent unstable response to or coupling with pressure fluctuations in the combustor.
To burn a fuel in air in a mixture beyond the flammable limit requires that the mixture be uniformly and intimately mixed and preheated to a temperature above the auto-ignition temperature. It also requires the combustion reactions to take place uniformly throughout the mixture, with no need or dependence on flame propagation. Conventional devices such as an igniter are useless. For example, the auto-ignition temperature with natural gas or methane fuels in air is about 1200° F., and the auto-ignition temperature with pulverized coal in air is about 500° F. Usually the air is strongly preheated, resulting in a final mixture that is above those temperatures.
In addition, when burning under very fuel-rich or fuel-lean conditions, the temperature and density of the combustion products vary significantly with the mixture ratio. Combustion chamber pressure perturbations change the injection system pressure drops and in turn, the total flow of fuel and air into the combustor is also changed. If the response of those flows to pressure perturbations is unequal, then the injected mixture ratio will vary as well. In such a case, the resulting combustion chamber pressure may reflect reductions not only in the total mass flows into the combustor but in the temperature and density of the combustion products as well. As a result, when burning very fuel-rich or fuel-lean, the probability of unstable feedback coupling is greatly increased.
Therefore, in order to protect against feed system coupled combustion instability, the fuel and air injection system must be designed such that the reactant in excess supply is more responsive to chamber pressure perturbations. The response to a chamber over-pressure would then be to change the mixture ratio in the direction of higher temperature, lower density combustion products. For example, when burning natural gas and air in very fuel-rich mixtures, gas is in excess supply and so the gas-side injection system should involve lower pressure drops and less inertance than the air-side injection system. In such an injector design an over-pressure will slow the gas flow more than the air flow, and the resulting mixture will be less fuel-rich (closer to stoichiometric), hotter and less dense, tending to compensate for the reduction in total flow.
Various prior art references have disclosed different injection schemes to mix one reactive fluid with another, for the purpose of combustion. The following are representative examples of such prior art:
1. U.S. Pat. No. 4,012,189 issued to Vogt on Mar. 15, 1977 for “Hot Gas Generator” (hereafter the “Vogt Patent”) disclosed a hot gas generator for the production of hot combustion gases includes a cylindrical combustion chamber having an inner and an outer air conduit concentrically disposed thereabout. A fuel nozzle means is arranged at one longitudinal end of the cylindrical combustion chamber and an exhaust port is arranged at the other longitudinal end thereof. A baffle plate is disposed on the longitudinal end of the cylindrical combustion chamber on which the fuel nozzle means is arranged. Combustion air from a blower or the like flows through the outer air conduit into the inner conduit where it is heated by the cylindrical combustion chamber and then passes through openings in the baffle plate into the cylindrical combustion chamber.
2. U.S. Pat. No. 4,297,093 issued to Morimoto on Oct. 27, 1981 for “Combustion Method for Reducing NOx and Smoke Emission” (hereafter the “Morimoto Patent”) disclosed a combustion method which can reduce the emission of NOx and smoke, by adopting a specific flow pattern of fuel and combustion air in the combustion chamber, which pattern was obtained as a result of studies and experiments concerning the influence of the intensity of mixing of the fuel and the combustion air on the emission of NOx.
3. U.S. Pat. No. 4,657,504 issued to Akiyama on Apr. 14, 1987 for “Combustion Burner” (hereafter the “Akiyama Patent”) disclosed a burner assembly of a type wherein both the combustion air and gaseous fuel are controlled by a pressure equalizing control device which has an inner barrel and a combustion air preheating passage both arranged exteriorly of a gaseous fuel supply tube, an annular space defined exteriorly of the gaseous fuel supply tube and communicating with the preheating passage through at least one perforation in the inner barrel, and an exhaust passage provided with an preheating passage.
4. U.S. Pat. No. 5,180,300 issued to Hovis on Jan. 19, 1993 for “Low NOx Regenerative Burner” (hereafter the “Hovis Patent”) disclosed a regenerative burner having heat storage units with combustion effluent/combustion air ducts there through, fuel intake means and a burner body, wherein the burner is designed to suppress NOx formation and to control flame shape and characteristic in the regenerative system during combustion. The regenerative burner may include a burner baffle, or may include a plurality of gas jets entrained in generally converging fashion for control of the flame characteristics and shape dispositive of NOx formation. The burner may provide for staged combustion, either by means of sequential fuel injection or sequential provision of combustion air, or the burner may depress NOx formation by vitiation of combustion air with products of combustion. The present regenerative burners suppress NOx formation yet preserve the remaining characteristic features of regenerative systems.
5. U.S. Pat. No. 5,359,966 issued to Jensen on Nov. 1, 1994 for “Energy Converter Using Imploding Plasma Vortex Heating” (hereafter the “Jensen Patent”) disclosed a heating system for heating a heat sink via a heat transfer medium. The invention includes a vortex chamber having opposite first and second inwardly curved end walls, a combustion chamber fluidly communicating with the vortex chamber, air-fuel supply means fluidly communicating with the combustion chamber for injecting air-fuel mixture into the combustion chamber. Ignition means are provided in the combustion chamber for igniting the air-fuel mixture. A fuel ionizing chamber is disposed in the vortex chamber fluidly communicating with the air-fuel supply means for ionizing fuel entering the air-fuel supply means, and heat transfer medium containing means are provided for holding the heat transfer medium in thermal contact with the vortex chamber.
6. U.S. Pat. No. 5,411,394 issued to Beer on May 2, 1995 for “Combustion System For Reduction Of Nitrogen Oxides” (hereafter the “Beer Patent”) disclosed a low NOx burner for the combustion of gaseous, liquid and solid fuels. The fluid dynamic principle of radial stratification by the combustion of swirling flow and a strong radial gradient of the gas density in the transverse direction to the axis of flow rotation is used to damp turbulence near the burner and hence to increase the residence time of the fuel-rich pyrolyzing mixture before mixing with the rest of the combustion air to effect complete combustion.
7. U.S. Pat. No. 5,460,513 issued to Flanagan on Oct. 24, 1995 for “Low NOx Burner” (hereafter the “Flanagan Patent”) disclosed a burner structure and a method of operating a burner to reduce the pollutant emissions produced thereby are disclosed. Air and gas are premixed in a manner such that a substantially homogeneous mixture containing excess combustion air results. The velocity of the substantially homogeneous mixture is increased as it passes through the burner causing the “residence time” associated with the formation of the flame to be decreased, i.e., the combustion gases are in the reaction zone of the flame for a significantly shorter period of time, reducing the production of NOx. In order to prevent the flame from “lifting-off” the burner because of the high velocity of the substantially homogeneous air/gas mixture, flame stabilizing devices and/or a burner structure which provides flame stabilization are utilized.
8. U.S. Pat. No. 5,846,067 issued to Nishiyama on Dec. 8, 1998 for “Low-NOx Burner” (hereafter the “Nishiyama Patent”) disclosed a low-NOx burner that is effective for reduction in NOx in a mid-temperature range which has been conventionally difficult to be realized and improves stability of the flame. In the low-NOx burner, at an outlet of an air throat for flowing a full quantity of the combustion air is disposed a burner tile having an enlarged diameter portion thereof whose diameter is larger than that of the outlet, and a fuel nozzle for injecting the fuel from the enlarged diameter portion of the burner tile is also provided. Further, a flow of the combustion air injected from the air throat produces a negative pressure at a secondary combustion chamber surrounded by the enlarged diameter portion of the burner tile around the air throat to cause a strong furnace exhaust gas recycle to occur, and a flame holding area, a furnace exhaust gas recycle combustion area and a slow combustion area are formed.
9. U.S. Pat. No. 5,861,600 issued to Jensen on Jan. 19, 1999 for “Fuel Plasma Vortex Combustion System” (hereafter the “First Jensen Patent”) disclosed a combustion system having a combustion chamber. The combustion chamber has a fuel inlet, a preheating chamber surrounding the combustion chamber, an air inlet for tangentially feeding combustion air to the preheating chamber, the combustion chamber having an elongated slot for tangentially admitting preheated air in circulating motion to the combustion chamber, a plasma chamber coupled to the combustion chamber having an inlet aperture for receiving combusting air-fuel plasma from the combustion chamber, and an outlet aperture for expelling combusted gas, the plasma chamber having an inverted end wall surrounding the outlet aperture operative for forming an imploding vortex in the plasma chamber.
10. U.S. Pat. No. 5,944,507 issued to Feldermann on Aug. 31, 1999 for “Oxygen/Oil Swirl Burner” (hereafter the “Feldermann Patent”) disclosed a liquid fuel burner that is provided with a central fuel outlet having a generally divergent conical inner surface, formed of two contiguous divergent conical surfaces of different angles of divergence, and a plurality of oxygen outlets shaped and positioned for creating a converging, rotating stream of oxygen which intersects with any liquid fuel issuing from the fuel outlet. Such oxygen/fuel interaction results in two zones of combustion and a recirculation effect which assists in the complete or substantially complete combustion of undesirable exhaust gas components. The oxygen and fuel are preferably supplied such that their velocities are approximately equal at the point at which the two zones of combustion meet. However, the Felderman Patent does not teach that the fluid streams must impinge and quickly mix or that the fluids must be preheated such that the initial temperature is above the autoignition temperature of the mixture. Rather, the Felderman Patent teaches a type of swirl-type mixing that depends upon adequate flame propagation throughout the mixture to complete combustion, a process inadequate or impossible in mixtures close to or beyond the flammable limit.
11. U.S. Pat. No. 5,961,312 issued to Sugiyama on Oct. 5, 1999 for “Combustion Burner and Combustion Method thereof in Furnace” disclosed a combustion burner that comprises: an air supply passage for supplying an air to a heating furnace; a primary fuel nozzle for supplying an air to a heating furnace; a primary fuel nozzle for supplying a primary fuel to the air supply passage; secondary fuel nozzles arranged around the air supply port of the air supply passage; and the secondary fuel nozzles being arranged so that a distance from an outer periphery of the air supply port to the outer periphery of the secondary fuel supply port is larger than the diameter of the air supply port. It also disclosed a combustion method that includes the steps of injecting fuel substantially from the primary fuel nozzle when an in furnace temperature of the heating furnace is lower than a fuel ignition temperature, and injecting fuel substantially from the secondary fuel nozzle when an in furnace temperature of the heating furnace is higher than a fuel ignition temperature.
12. U.S. Pat. No. 5,968,378 issued to Jensen on Oct. 19, 1999 for “Fuel Plasma Vortex Combustion System” (hereafter the “Second Jensen Patent”) disclosed a combustion system having a combustion chamber having a fuel inlet, a preheating chamber surrounding the combustion chamber, an air inlet for tangentially feeding combustion air to the preheating chamber, the combustion chamber having an elongated slot for tangentially admitting preheated air in circulating motion to the combustion chamber, a plasma chamber coupled to the combustion chamber having an inlet aperture for receiving combusting air-fuel plasma from the combustion chamber, and an outlet aperture for expelling combusted gas, the plasma chamber having an inverted end wall surrounding the outlet aperture operative for forming an imploding vortex in the plasma chamber.
13. U.S. Pat. No. 5,984,667 issued to Philippe on Nov. 16, 1999 for “Combustion Process and Apparatus therefore Containing Separate Injection of Fuel and Oxidant Streams” (hereafter the “Philippe Patent”) discloses a burner assembly having improved flame length and shape control which includes in exemplary embodiments at least one fuel fluid inlet and at least one oxidant fluid inlet, means for transporting the fuel fluid from the fuel inlet to a plurality of fuel outlets, the fuel fluid leaving the fuel outlets in fuel streams that are injected into a combustion chamber, means for transporting the oxidant fluid from the oxidant inlets to at least one oxidant outlet, the oxidant fluid leaving the oxidant outlets in oxidant fluid streams that are injected into the combustion chamber, with the fuel and oxidant outlets being physically separated, and geometrically arranged in order to impart to the fuel fluid streams and the oxidant fluid streams angles and velocities that allow combustion of the fuel fluid with the oxidant in a stable, wide, and luminous flame. The burner assembly affords improved control over flame size and shape and may be adjusted for use with a particular furnace as required.
It appears that none of above cited prior art references specifically teaches the special operating conditions necessary for stable operation under very fuel-rich or very fuel-lean conditions. Nor do any of the above cited prior art references teach that the temperature and density of the combustion products are strong functions of the ratio of those reactants, under those extreme combustion conditions. Neither does any of the above cited prior art references teach the strong negative impact this can have on combustion stability.
Moreover, it appears that the above cited prior art references are designed to burn air-fuel mixtures very close to stoichiometric, and none has addressed the challenges of burning fuel with an oxidizer under the difficult combustion conditions of very fuel-rich and fuel-lean mixtures. As a result, none of the above cited prior art references recognizes the need to create a sound, solidly anchored flame and stable combustion under conditions which may be very close to or even beyond the flammable limit for those mixtures.